Being a return you did not prepare you are preparing the return again only to get a refund of $50.  Could they not amend the return themselves? 

If it is a return I have prepared and not much work involved honestly I do not charge.  Depending on the scope of the work would determine the cost but usually not more than $100.  If it is a return they prepared and I am amending I charge the same as I would for the original return.

We are a small bookkeeping and tax office.  I do all the bookkeeping for one customer and someone else prepares his return.  I found an error not on my part but something the customer did not tell me.  I informed the CPA, he amends the return and charged them $350 for a simple change.  Kinda crazy for me.